Homogenous Charge Compression Ignition (HCCI) is an advanced combustion concept for piston engines that offers significant efficiency and emissions benefits over current technologies. The HCCI combustion process has been studied for over two decades, and has shown promise as a potential technology for automotive engines that can improve on the efficiency and emissions capabilities of current technologies.
In HCCI, a homogeneous mixture of air, fuel and hot exhaust gases is compressed until auto-ignition occurs. Consequently, combustion is not initiated by a spark. Rather, precise conditions are established within a cylinder such that simply by compressing the gases within the cylinder ignition is initiated. HCCI is thus highly dependent upon the in-cylinder temperature and composition of gases. In order to provide a desired temperature for HCCI ignition, a significant amount of hot exhaust gas from the previous combustion cycle is typically trapped within the cylinder to enable this auto-ignition; however, other methods for initiating HCCI have also been tested, including increasing the compression ratio and heating the intake air.
A significant benefit to incorporating HCCI is that HCCI engines can be run fully unthrottled, significantly reducing pumping losses that are typical in a spark-ignited (SI) engine, thereby boosting the efficiency. Additionally, due to the highly diluted reactant mixture and absence of a flame, peak combustion temperatures are much lower, which reduces NOx emissions significantly.
The provision of an HCCI only system is problematic, however, because of load limitations. Auto-ignition occurs with very high pressure rise rate leading to the phenomenon of ringing at higher loads which is structurally undesirable for the engine. Hence there is a cap on the maximum power output in HCCI. At the low load end, HCCI mode is harder to maintain because the temperature required to auto-ignite cannot be achieved. HCCI mode is also not possible at lower speeds as the chemical breakdown of species to initiate auto-ignition is at a very slow rate at lower speeds. This leads to unstable operation or misfire.
Therefore there exists only a limited operational region for running an engine efficiently and stably in HCCI mode. Accordingly, attempts have been made to incorporate HCCI mode in an automotive engine by combining it with the conventional SI mode. In these approaches, SI mode is used during cold startup periods and during ramping up the engine through low-speed and low loads. In the region of medium to medium-high loads, the engine can be operated in HCCI mode, maximizing the efficiency and minimizing emissions. The mode can be switched back to SI when the power demand exceeds the upper-load limit of the HCCI mode.
Transitioning smoothly from one mode to another however, presents additional challenges. For example, maintaining a desired torque during mode switching can be challenging due to the significant differences between SI and HCCI operating conditions. Therefore, implementation of HCCI on a production engine requires advanced control algorithms. The control algorithms are complicated due to the lack of a direct ignition trigger (such as a spark), and the cycle-to-cycle dynamics introduced by the trapped exhaust gas in an HCCI mode. Several modeling and control approaches for steady-state and transient control of HCCI have been presented in the literature.
As evident from the foregoing discussion, transitions between HCCI and traditional SI mode are necessary both at the low load/speed as well as the mid-high load/speed end of the operating range. This is shown schematically in FIG. 1. In FIG. 1, the region of engine loads/engine speeds wherein HCCI mode is advantageous is indicated by area 10. The area 12 identifies the allowable operating region of SI mode Accordingly, as an engine transitions along a line 14 from a low speed/low load condition to a high speed high load condition, the engine will optimally transition from SI mode to HCCI mode at location 16 and transition from HCCI mode to SI mode again at location 18. Similarly, as an engine transitions along a line 20 from a high speed/high load condition to a low speed/low load condition, the engine will optimally transition from SI mode to HCCI mode at location 22 and transition from HCCI mode to SI mode again at location 24.
To accomplish the desired mode switching, HCCI engines are typically implemented either with a fully flexible variable valve actuation system, or with dual cam phasers. The former is suitable only for research purposes, and is not feasible to implement on a production setup. Dual cam phasers for HCCI engines are typically designed with two sets of valve profiles, one for SI mode and one for HCCI mode.
FIG. 2 shows a typical example of SI and HCCI valve lift and open/close profiles. Line 30 identifies the valve lift position of the exhaust valve versus crankshaft angle degree (CAD) in an SI mode and the line 32 identifies the valve lift position of the intake valve versus CAD in an SI mode. The line 34 identifies the valve lift position of the exhaust valve versus CAD in HCCI mode and the line 36 identifies the valve lift position of the intake valve versus CAD in HCCI mode. The maximum heights of the lines 30, 32, 34, and 36 are referred to herein as “valve lift profiles”, while the CAD at which the lines depart and return to 0 mm is referred to as the valve “open/close” or “timing” profile.
FIG. 2 shows that in SI mode, it is desirable to have high valve lifts (lift profile), and long open valve durations (open/close profile), while in HCCI mode it is preferable to have low valve lifts and short open valve durations (to allow for trapping of exhaust gases). Consequently, a transition from SI to HCCI also involves a switch from the SI valve profiles (both lift and timing profiles) to the HCCI valve profiles, which can introduce significant dynamics.
Different approaches for switching between the two modes have been presented in the literature, including single-step switches as well as transitions that happen more gradually over several cycles. Some control approaches have also been presented in the literature—one article shows a control approach to switch from SI with early intake valve closing to HCCI, where fuel quantity and valve timings are controlled during the switch; another article presents an approach to control load and air fuel ratio during a multi-cycle transition from SI to HCCI. All these methods, however, result in undesired dynamics.
What is needed therefore is a control system which provides for transitions between SI and HCCI modes while exhibiting improved dynamics.